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Re: Must tell Compile that Clip returns a number.

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  • Subject: [mg107748] Re: Must tell Compile that Clip returns a number.
  • From: Albert Retey <awnl at>
  • Date: Thu, 25 Feb 2010 01:54:07 -0500 (EST)
  • References: <hm3210$m7a$>


> (v7.0.1) I don't know whether this would be considered a bug or not,
> but it took me a bit to figure out what was going on, so someone else
> might find it useful.

I think it is no bug, it just is one of the many inconveniences of the
current Compile functionality....

> For some reason, Compile seems to expect Clip to return a list. (Maybe
> because it takes a list argument.) For example:
> In[1]:= Compile[{x}, Clip[x, {0, 1}]][0]
> CompiledFunction::cfte:
>    Compiled expression 0. should be a rank 1 tensor of
>     machine-size real numbers.
> CompiledFunction::cfex:
>    Could not complete external evaluation at instruction 3; proceeding
> with
>      uncompiled evaluation.
> Out[1]= 0
> A workaround is to use Compile's third argument to tell it that any
> expression with head Clip will evaluate to a single number:
> In[2]:= Compile[{x}, Clip[x, {0, 1}], {{_Clip, _Real}}][0]
> Out[2]= 0.

While that saves you from the error message you will very likely be
disappointed with the performance. If you look at the InputForm of the
compiled function you will notice that the second argument is a nested
list of numbers and one occurance of Clip. This means that the code
needs to call back to the regular evaluation process to evaluate Clip
which usually causes the compiled code to be hardly faster than the
uncompiled code, in some cases it will even be slower. To really want to
see speedup from Compile you need to take care that you only use
constructs that can be compiled. Looking at the InputForm you will find
that for the following only numbers appear in the nested list in the
second argument which indicates that the whole body could be compiled
for that case:

Compile[{x}, Max[Min[x, 1], 0]] // InputForm

Unfortunately all this seems to be documented only superficially and
there is no known method but trial and error to determine which
constructs can be compiled, but you should at least study this:




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